Strip-line power dissipative device

ABSTRACT

A STRIP-LINE POWER DISSIPATIVE DEVICE IN WHICH THE GROUND PLATE OR PLANES ARE MADE RESISTIVE WHEREBY THE POWER WILL BE DISSIPATED IN THE GROUND PLANE OR PLANES.

Feb. 16,1971 '4 co ETR AL 3,564,464

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IINVENTORS 'i' K.E. HANCOCK .I. BULLEN m i &

United States Patent O 3,564,464 STRlP-LINE POWER DISSIPATIVE DEVICE Kenneth Edney Hancock, Roxboro, Quebec, and James I. Bullen, Mount Royal, Quebec, Canada, assigno's to ABSTRACT OF THE DISCLOSURE A stripline power dissipative device in which the ground plane or planes are made resistive whereby the power will be dissipated in the ground plane or planes.

This invention relates to a device for power dissipation. More specifically, this invention relates to a strip line device which uses the ground plane or planes as the dissipative element.

In electrical circuits calling for strip or coaxial transmission lines, it is often necessary to terminate the circuit with a high power load having an impedance equal to the characterstic impedance of the strip or coaxial transmission line. In the known method of constructing such a device, the center conductor is made of a resistive material, which is appropriately tapered for impedance matching, and the power is dssipated as heat in the center conductor.

The disadvantage of these known devices is that it is dflicult to transfer the heat efciently from the center conductor to the outer surface of the device from which it is to be removed as by radiation, convection or conducton.

It is an object of the invention to provide a device which overcomes the above disadvantage.

It is a further object of the invention to provide a device in which the thermal transfer to the outer surface of the device is more efiicient than in the known devices.

It is a still further object of the invention to provide a device which allows greater power dissipation for a given size device than is presently possible.

According to the invention, a strip line power dissipative device is provided which uses the ground plane or planes as the power dissipative elements.

The invention will be better nnderstood by examination of the following description together with the drawings in which:

FIG. 1 is a side view of one embodiment of the invention;

FIG. 2 is a section through Il-II of FIG. 1; and

FIG. 3 is a side view of a second embodiment of the invention.

It is well known in the art that a transmission line may be formed by placing a conductor between two infinite parallel planes referred to in the art as ground planes. Such a construction is the so-called strip transmission line and the center conductor is supported between and spaced from the ground planes by a dielectric material either along the entire surface of the ground planes or by dielectric posts which support the center conductor -at preselected points. The impedance of such a strip transmission line is proportional to the geometry of the center conductor and the dielectric thickness.

It is also well known in the art to construct such a strip transmission line using only one ground plane.

In accordance with the principle of the invention, a circuit termination comprises a strip line power dissipative device wherein the ground planes are made of a resistive material. In the preferred embodiments, for im- "ice pedance matching purposes, the spacng between the center onductor and each ground plane is exponentially decreased over the length of the resistive ground planes from the input end of the device such that, at the end remote from the input end, the spacing between the :center conductor and each ground plane is zero, i.e. the center conductor makes contact with the ground planes. However, it is stressed that this is not a necessary feature of the invention, and devices have been produced where the spacing between the center conductor and the ground planes remains constant. The decrease in spacing may be accomplished by increasing the thickness of the center conductor exponentially along the length of the ground planes, whereby the device can act as a matched load absorbing all the power fed into it.

By making the ground planes resistive, substantially all of the power will be absorbed into the ground planes making thermal transfer to the outer surfaces extremely efficient and allowing greater power dissipaton than was formerly possible, When using two ground planes, the resistivity of each ground plane may be made equal to twice the characteristics impedance of the transmission line. When only one ground plane is used, the size and shape of the resistive material of the ground plane may be such that the resistance of said material is equal to the characteristic impedance of the transmission line of the circuit.

Referring now to FIG. 1, a center conductor 1 is shown disposed 'between two ground planes designated generally as 2. The conductor may be any suitable conductive material such as aluminum, Copper or silver, although in the preferred embodiment, aluminum is used. Each ground plane 2 comprises a ceramic plate 3, such as alumina, beryllium oxide or boron nitride, contact areas 4, and a resistive area 5, the contact and resistive areas being mounted on each ceramic plate on the surface of the plates adjacent the center conductor such that the ceramic plates constitute the outside surface of the device. The contact area may be gold or copper and the resistive material may be silver film, carbon film or Nichrome. In the preferred arrangement, the contact area is gold and the resistive material used is Nichrome. Contact is made with the circuit at the input of the device 6.

The resistive material 5 is preferably of such a size and shape that the 'DC resistance between the Contacts 4a and 4b is 2Z0 for each ground plane where Zo is the characteristic impedance of the circuit transmission line. As can be seen in the drawing, the planes are spaced from each other and disposed in parallel arrangement one on top of the other.

At the input end, the 'width of the center conductor and the spacing between the ground planes is such that the input inpedance of the device matches the line impedance. The thickness of the center conductor is then increased exponentially until it makes contact with each ground plane at the end remote from said input end as at 7.

The device may then be placed in a suitable enclosure, and by the proper selection of dimensions of strip thickness and width, taper curve and spacing between the center conductor and the ground planes, according to principles which are well known to one skilled in the art, it is possible to adjust the load to give good impedance matches over a frequency range of several octaves.

FIG. 2 is a section through II-II of FIG. 1. Like numerals refer to like parts relative to FIG. 1.

The embodiment using only one ground plane is illustrated in FIG. 3. Here again, like numerals 'efer to like parts relative to FIG. l. In the embodiment shown in FIG. 3, the DC resistance of the resistive area 5 is preferably equal to ZO. The thickness of the center conductor at the input end of the device and the spacing between the center conductor and the ground plane is such as to give the device an input impedance equal to Zo.

It will be obvious to one skilled in the art that this same princple could be used for devices other than terminations. For example, an attenuator could be constructed using the same basic idea of using a resistive material to fabricate the ground plane, and thus having the dissipated power absorbed in the ground plane.

Although several embodiments of the invention have been described in the foregoing, it is to be understood that these are for purposes of illustration only and are not meant to be restrictive. Various modifications which will readly come to the minds of those skilled in the art are all considered to come within the full scope of the invention as defined in the appended claims.

We claim:

1. A power dissipative device of the strip line type for transmisson lines having a characteristic impedance ZO, said device having an input end and an end remote from said input end and further comprising; at least one substantially flat ground plane element comprisng a resistive material; contact areas disposed at the input end and at the end remote from said input end and connected to said resistive material; a conductive element disposed along the length of said ground plane element and spaced from said ground plane element at the input end of said device; wherein the ground plane element is of such a size and shape that the DC resistance of the ground plane is substantially equal to nZo, wherein n is the number of ground planes and is equal to one or two; and wherein the width of said conductive element at the input end of said device, and the spacing of said conductive element from said ground plane at the input end of said device is such as to provide an input impedance to said device equal to ZO, and wherein the spacing between said conductive element and said ground plane element is decreased along the length of said ground plane element from said input end to said end remote from said input end, and wherein said decrease in spacing is an exponential decrease, and wherein the resistive material is selected from the group consisting of silver film, carbon film and Nichrome and wherein the resistive material of the ground plane element is mounted on a heat conductive ceramic plate of material selected from the group consisting of alumina, berylwherein the material of said conductive element is selected from the group consisting of aluminum, Copper and silver.

4. A power dissipative device as defined in claim 3 wherein said resistive material comprises Nichrome, said contact areas are formed of gold, said center conductor comprises alumnum and wherein the resistive material of said ground plane is mounted on an alumina plate on the surface of said plate adjacent said conductive element such that the ceramic plate forms the outside 'surface of said device.

5. A power dissipative device as defined in claim 4, comprisng two ground plane elements said planes being spaced from each other and disposed in parallel arrangement one on top of the other, said conductive element being placed in the space between said plates and half way between them, wherein the Nichrome material of each of said ground planes is mounted on an alumina plate on the surface of said plate adjacent said center conductor such that the alumina plates constittue the outside surfaces of the device, the DC resistance of each ground plane element being to equal 2Zo, and in which the width of said conductive element at said input end of said device and the spacing between the ground plane elements and said conductive element at the input end of said device is such as to provide an input impedance to said device equal to Zo.

References Cited UNITED STATES PATENTS 2,810,891 3/1954 Englemann 333-81 2,725,S35 5/1951 Grieg 333-84M 2,961,621 11/1960 Tannenbaum et al. 333-81 2,909,736 10/ 1959 'Sommers et al. `3 3--81 3,2.-l5,958 11/ 1965 Isaacson 333-84M 3,341,79O 1-2/1967 Candilis 333--81 `3,309, 634 3/ 1967 Wheeler 333-81 FOREIGN PATENTS 839,208 6/ 1960 Great Britain 333-84M HERMAN KARL SAALBACH, Primary Examiner C. BARAFF, Assistant Examiner U.S. Cl. X.R. 333-22; 84(M) 

